1. Field of the Invention
Ammonium phosphate is effective for fertilizer use because of its favorable physical properties, high concentration of plant foods, and because it provides an economical method of fixing ammonia in a solid form. This material is attractive because it is a multinutrient fertilizer that has been demonstrated to be a very effective source of the major plant nutrients nitrogen and phosphorus. Agronomic data show that ammonium phosphate is particularly suited for use in acid soils when residual acidity thereof has been neutralized.
Monoammonium phosphate has been produced commercially in the United States, Canada, and European countries, such as England and Spain, principally from the reaction of ammonia and wet-process phosphoric acid by processes such as those that were developed by the Imperial Chemical Industries of England and the Scottish Agricultural Industries of Scotland; however, products produced by both of these processes are nongranular and are dusty and, for this reason, are impractical for use as a direct application material or for the production of bulk blends. Products produced by the practice of the process of the instant invention will be dust free and of such particle size distribution that they are well suited for the production of bulk blends and direct application.
2. Description of the Prior Art
In U.S. Pat. No. 3,153,574, Achorn et al disclose a process for the manufacture of diammonium phosphate from wet-process phosphoric acid and ammonia; however, this process requires the use of a preneutralization step which, in turn, requires a large preneutralizer tank. A principal advantage of the instant invention is that this requirement of Achorn et al of the preneutralization step and the large preneutralizer tank has been eliminated. In addition, our process eliminates the need for a separate step to dry the solid ammonium phosphate products. Elimination of the expensive and difficult drying step is important in that it decreases investment and operating costs, as well as substantially decreasing energy requirements. Pollution abatement considerations associated with these teachings of Achorn et al are greatly simplified by following the teachings of the instant invention since the greatest source of fumes and dust in Achorn et al supra is eliminated by the process of the instant invention.
In U.S. Pat. No. 2,902,342, Kerley et al, a process is described for the production of ammonium phosphate sulfate. However, in Kerley's process, both a complicated acid preparation and a first ammoniation step are required. The first ammoniation step is quite similar to the preneutralization step described by Achorn et al supra. A primary objective of the instant invention is to eliminate these extra steps and the complicated and expensive equipment thereby required. We have discovered that this objective can be accomplished by using a specially designed pipe-cross reactor to effect the neutralization of the acid.
In U.S. Pat. No. 3,985,538, Hicks et al disclose a process for the production of granular ammonium polyphosphate fertilizer in which the ammonium polyphosphate melt is prepared in a pipe reactor and dispersed through a series of apertures in the wall of the pipe to bind small particles of fertilizer into granules. However, a preneutralization step is required. This step is also quite similar to the preneutralization step described by Achorn et al supra. The practice of the instant invention eliminates this step thought to be necessary by Hicks et al as well as the extra reaction, transfer, and metering equipment therein required.
In U.S. Pat. No. 4,134,750, Norton et al disclose a process for production of ammonium phosphate, ammonium phosphate sulfates, and urea-ammonium phosphate sulfates using a well-engineered pipe-cross reactor. They introduce sulfuric acid to this pipe-cross reactor in order to obtain up to about 30 percent polyphosphate in their end product. In doing so, they have to add copious amounts of sulfuric acid in order to initially make about 60 percent polyphosphate in the melt produced in their pipe-cross reactor. This is necessary in that they retain only about one-half of this polyphosphate in their final granular product (see Example III, test PCX-6). In other words, the hydrolysis of the polyphosphate P.sub.2 O.sub.5 to orthophosphate P.sub.2 O.sub.5 is very substantial. In addition, their costs to obtain an end product having 30 percent polyphosphate are very high. For instance, there is the raw material cost of excessive amounts of sulfuric acid used, and the high costs of the materials of construction of the equipment required since the use of such amounts of sulfuric acid provides a highly corrosive environment. In the process of the instant invention, we can produce an end product of granular ammonium polyphosphate containing 27 percent polyphosphate with the melt produced in our pipe reactor containing also only about 27 percent polyphosphate P.sub.2 O.sub.5. Our process retains a higher proportion of the polyphosphate in the granular product without use of sulfuric acid than that made by Norton et al supra and any known conventional pipe reactor-drum granulation process of ammonium polyphosphate where sulfuric acid is used. Also, our product is harder, rounder (sphericity is about 30% or greater) and has excellent physical properties that are well suited for both bulk blends and high-quality suspensions because it contains 20 percent to 30 percent of the total P.sub.2 O.sub.5 as polyphosphate that has layers of metal phosphates and metal impurities on the seed recycle.
In U.S. Pat. No. 4,337,079, Mann et al describe an enlarged pipe-cross reactor which is similar to the one used in the instant process whereby they produce melts containing about 20 percent to about 30 percent polyphosphate without use of sulfuric acid. They do not try to granulate their melts but rather produce suspensions from them. Their enlarged pipe reactor is operated under pressures less than 20 psig i.e., their pipe reactor is submerged under liquor in order to prevent ammonia loss. The pressure they operate their reactor under is that encountered by the raw materials that is fed to the reactor, whereas we pressurize our enlarged pipe reactor in order to fix all the ammonia as well as use it to spray melt onto seed recycle in the granulator.
In particular, the present invention relates to granulation in an enclosed vessel, such as, for example, a rotating drum granulator, wherein air is or can be ventilated therethrough, and wherein said air does or can be made to come in contact with the material therein granulated and/or the wall or internals of the vessel which contact said materials such that the air can be used to either directly or indirectly transfer heat from the material which is therein solidified and cooled.
In many processes for granulation of mixed fertilizer previously known to the art, such as those taught in both U.S. Pat. No. 2,926,079, Smith, and U.S. Pat. No. 2,798,801, Kieffer et al, a drying step is required to remove excess moisture. The process of the present invention eliminates the need for such a drying step. The process of the instant invention also eliminates the need for a pressurization step as well as the requirement for special equipment, such as the dehydration chamber, disclosed in U.S. Pat. No. 3,415,638, Hamsley et al.
One of the classical methods of granulating is gas prilling wherein droplets of molten material are formed by any number of means and are allowed to fall through tremendous volumes of gas flowing countercurrent thereto. Heat is removed by the flowing gas allowing the granules to solidify. Examples of some such processes are described by Williams et al (U.S. Pat. Nos. 2,402,192 and 2,774,660) wherein respectively a 95 percent aqueous solution of ammonium nitrate and a substantially anhydrous, ammonium nitrate melt were the sprayed materials. Gas prilling is now one of the foremost granulation processes worldwide, but it has many disadvantages, one of which is the tremendous amount of cooling air required. Also, the cost of construction for this type of plant is high. Prills are inherently small and, for most materials, contain voids and/or surface dimples.
Other researchers have disclosed methods of reducing the tremendous volume of countercurrent gas flows in shot towers by external cooling loops such as that of Jewett et al (U.S. Pat. No. 1,837,869) wherein the gas is passed through a cooler supplied with cooling coils in which a brine solution circulates and precools the air before it enters the shot tower, thus in effect reducing somewhat the size of tower and quantity of cooling air required, but with the attendant expense of installing external coolers.
In further similar disclosures, Ishizuka et al (U.S. Pat. No. 3,058,159) and Klopf (U.S. Pat. No. 3,231,640) advocated introducing water in the form of a spray or mist in the incoming air at the bottom of the shot tower. The water droplets would be small enough to be carried up the shot tower and evaporate as they come into contact with the larger falling particles of granulating material. However, because of the direct contact of the water droplets and the solidifying particles, this method of heat removal is not practical for practice of the process of the instant invention wherein it is desirable that the product be discharged from the shot tower dry, such as in sulfur granulation; where the granulating material is hydroscopic, such as in urea granulation; or where direct contact of water and granules is prohibited for any reason chemical or physical. For those knowledgeable in the art, it is obvious that the teaching of Jewett et al, Ishizuka et al, and Klopf supra could be combined such that precooling is done by evaporation of water external of the shot tower such as in a humidifying chamber and then introduced to the tower as a somewhat precooled humid air free of water mist. Those knowledgeable in the art will also appreciate that precooling by this nature is limited severely in humid climates and almost useless when granulating hydroscopic products such as urea.
Disclosures made by Bottai et al (U.S. Pat. No. 3,578,433) and Campbell (U.S. Pat. Nos. 3,334,159 and 3,550,195) among others, advocated prilling into a liquid in which the prilled material was either insoluble or only slightly soluble. Bottai's invention involved prilling urea-ammonium polyphosphate in various liquid mediums. Campbell's disclosures involved prilling sulfur into water. These liquid cooling processes eliminated the use of gas and its associated problems but resulted in other problems. The prills retained a portion of the cooling medium which either had to be removed by other means, such as heating and drying, or had to be accepted as impurities in the product. Prills from liquid granulation were inherently small and still contained voids and/or surface dimples.
In U.S. Pat. No. 3,398,191, Thompson et al disclose a granulation process in which urea seeds are charged to the bed formed in a rotary drum. As the drum rotates, flights raise the solid particles from the bed and shower them down throughout the cross section of the drum. Urea is sprayed onto the cascading granules to build the granules in size. Air is drawn countercurrent to the product flow through the cooling section of the drum and then to the granulating section. No supplemental cooling external of the rotary drum was used in the example given, but their disclosure indicated that it could be employed as it was in the granulation of ammonium nitrate. In a somewhat similar disclosure in U.S. Pat. No. 3,877,415, Blouin describes a rotary drum process for applying coating to solid particles in which, by example, he shows its effectiveness as a coating or granulating apparatus in spraying sulfur onto a substrate as it falls in a continous curtain at a predetermined distance away from a number of spray nozzles. Blouin briefly proposes three possible ways of removing the heat given off by solidification of the sprayed material. Cooling gases could be passed through the granulating drum to directly contact the granules and absorb the heat; the material to be solidified might be sprayed in the form of a solution; and the evaporation of the solvent into a hot air stream could remove the heat of crystallization, or as he states, "by maintaining the particulate feed at a sufficiently low temperature that the resulting product emerges at a temperature below the fusion temperature of the coating medium." Those familiar with the art of granulation recognize that the latter can be done by recycling material through the drum while cooling the material external of the drum.
Rotary drum granulation processes make it possible to produce granules with improved physical characteristics over prilled materials, as is shown by example in the disclosure of Thompson et al. However, cooling is still a problem generally requiring the use of large quantities of cooling air or the recycle of large quantities of externally cooled granular material, both of which are expensive energy consuming ways of removing the heat associated with the granulation.
Sulfur slating as disclosed by Ellithrope and Fletcher in U.S. Pat. Nos. 3,885,920 and 3,838,973 allows the use of relatively inexpensive cooling medium water without the inherent problems associated with water prilling. This process is one of the primary methods of sulfur granulation employed at present. However, the product resulting from this process is not resistant enough to breakage and abrasion to allow it to meet some air pollution and safety standards which will become effective in the near future.
Berquin discloses in U.S. Pat. No. 3,231,413 a granulation process using a modified fluidized bed whereby a liquefiable material is injected into an incoming fluidizing gas and results in the impaction of particles of the liquefiable material onto the fluidized granules, thus gradually building them in size. Berquin's disclosure teaches that water can also be injected into the gas flow along with the liquefiable material provided the liquefiable material is not hygroscopic. According to the disclosure, the water impinges on the flowing bed of granules where it is immediately vaporized as steam and maintains the gaseous flow stream at 100.degree. C., thus removing heat from the process.
Sulfur has been commercially granulated by the Berquin process supra; whether or not these plants have used the evaporative cooling step is not known. Some sulfur processors considering the use of the Berquin process have noted the added expense which may be associated therewith because of high electrical power consumption necessary for the maintenance of the fluidized bed.
In U.S. Pat. No. 3,936,534, Schallis disclosed a blend of the rotary-drum granulation type process and water cooling. The water is atomized directly on a rolling bed which has no lifting flights as opposed to the disclosures of Thompson et al and Blouin supra. Sulfur is also fed directly to the bed, and the heat of solidification and cooling is primarily removed by water cooling. Air is used to promote drying in some instances. Schallis's invention, however, like Berquin's water cooling disclosure supra, appears suitable only for adaptation on hydrophobic materials, such as, for example, sulfur.
Shirley, U.S. Pat. Nos. 4,213,924, 4,424,176, and 4,506,453, uses relatively expensive equipment in his falling curtain method whereby he adds water for cooling and fans installed, as well as utilized inside the granulating drum to remove heat. In the process of the instant invention, we use lifting flights and inclined pan(s), but we do not require water addition and electric fans to evaporate water and cool the product. Cooling in our process is very important, but is simple in that when the melt and steam discharges from the enlarged pipe reactor the nominal air flow removes the steam leaving essentially an anhydrous melt for coating the seed recycle. Most of the heat is removed simultaneously with the steam.
Prior art arrangements in the processes of the above-mentioned types have proved to be operative; however, such processes require the expenditure of substantial amounts of capital for such relatively expensive equipment as large preneutralization tanks and acid preparation tanks, along with their associated piping, transfer, metering equipment, and fans for removal of heat.